Rotary wave-guide structure including polarization converters

ABSTRACT

A waveguide structure including at least one rotating joint between waveguides of circular cross-section is provided, on opposite sides of the joint, with groups of axially spaced elongate metallic elements which are arcuately curved in transverse planes of the circular waveguides and act as inductive differential phase shifters for microwaves to be transformed from linear to circular polarization or vice versa, the linearly polarized waves being fed in and taken off via adjoining waveguides of rectangular cross-section whose major surfaces include angles of 45* with chords spanning the arcuate elements. These elements may be divided into several groups spaced a quarter wavelength apart, with a larger separation between the elements within each group. They may also be alternately disposed at diametrically opposite locations in the circular waveguide.

United States Patent Levaillant et al.

[ 51 Sept. 16, 1975 ROTARY WAVE-GUIDE STRUCTURE INCLUDING POLARIZATION CONVERTERS [75] Inventors: Claude Levaillant; Andre Bensussan,

both of Paris, France [73] Assignee: CGR-Mev, Paris, France [22] Filed: Jan. 11, 1974 [21] Appl. No.: 432,689

[30] Foreign Application Priority Data Jan. 16, 1973 France 73.01455 {52] US. Cl 333/21 A [51] Int. Cl ..H01p l/15;H01p 1/16 [58] Field of Search 333/21 A, 98 TN [56] References Cited UNITED STATES PATENTS 2,432,093 12/1947 Fox 333/98 R 3,668,567 6/1972 Rosen 333/98 TN Primary Examiner-Alfred E. Smith Assistant ExaminerWm. H. Punter Attorney, Agent, or Firm-Karl F. Ross; Herbert Dubno [57] ABSTRACT A waveguide structure including at least one rotating joint between waveguides of circular cross-section is provided, on opposite sides of the joint, with groups of axially spaced elongate metallic elements which are arcuately curved in transverse planes of the circular waveguides and act as inductive differential phase shifters for microwaves to be transformed from linear to circular polarization or vice versa, the linearly polarized waves being fed in and taken off via adjoining waveguides of rectangular cross-section whose major surfaces include angles of 45 with chords spanning the arcuate elements. These elements may be divided into several groups spaced a quarter wavelength apart, with a larger separation between the elements within each group. They may also be alternately disposed at diametrically opposite locations in the circular waveguide.

l0 Claims, 14 Drawing Figures PATENTEI] SEP l 8 I975 SHEET 1 BF 3 PRlOR ART PATENTEBSEP I 81875 3.906 ,407 sum 2 or 3 PATENTED SEP I 8 I975 sumsqm ROTARY WAVE-GUIDE STRUCTURE INCLUDING POLARIZATION CONVERTERS Certain kinds of microwave equipment, such as radar or therapeutic irradiation apparatus 'for' example, incoi' porate a fixed section and a mobile section. It is then' desirable to transmit the microwave energy from the fixed section, to the mobile section or vice-versa through a waveguide which introduces no more than very minor disturbancesin the transmitted signal.

The rotating element which is included in the transmission circuit conventionally employs dielectric fins arranged at the center of a circular waveguide, these fins having the well known properties of polarizing high frequency waves. However, these dielectric fins are fragile and, moreover, 'rnay developbreakdown phenomena whose'magnitude depends upon the transmit ted power and which always adversely affect the life time of the device. Added to these drawbacks is the difficulty of efficiently cooling these dielectric fins, which may reach very high temperatures. i

It is'an object of the invention to overco drawbacks. k

A waveguide structure according to our'inven'tiori designed to convert linearly polarized (or planepolarized) microwaves into circularly polarized micro waves' and vice versa, comprises two waveguides of circular cross-section centered on a common axis, these circular waveguides being relatively rotatable about their axis th virtue of the interposition of a rotary joint enabling uninterrupted microwave transmission therebetween. An input waveguide of rectangular crosssection supplies the first circular waveguide with linearly polarized microwave energy having an electric field vector in a first radial plane including the waveguide axis; similarly, an output waveguide of rectangu-x lar cross-section extracts linearly polarized microwave me I these energy from the second waveguide, the extracted en-" ergy having an electric field vector in a second radial plane including that axis. The first waveguide isprovided with first inductive phaseshifting means comprising a first plurality of axially spaced elongate con'- duetive elements'of arcuate configuration connected at their ends to the inner peripheral surface of that waveguide and spaced therefrom between these ends while turning their concave sides toward the axis, a line spanning the ends of any element of this first plurality including an angle of substantially 45 with the aforementioned first radial plane. The second circularwaveguide is provided with a second plurality of similar axially spaced arcuate elements whose ends are spanned a line including an angle of substantially 45 with the sec ond radial plane. In this way, a component of the electric field vector of an incoming linearly polarized wave is phase'shifted with reference to another component perpendicular thereto to, provide. circular polarization ahead of the rotary joint; analogously, beyond that joint, the circularly polarized wave is reconverted by a similar phase shift into a linearly polarized outgoing. wave. I V

Instead of ,using ineon ingand ou guides, we may replace each of the se rectangular wav-' tgoing rectangular waveguides centered on the axis of the rotary ,waves i Furthermore, the rotary joint between .the first and second circular waveguides could be:replaced by apair ofi'axially spacedjoints separatedbya further wave now be described indetailwith reference to theaccompariying drawing wherein:w

FlGqil illustrates .aconventional application of a rotating-joint waveguide structure;- FIG. :2 schematically illustrates arotating joint waveguide structure, of conventional design;

. FIGS. 3 and 4 respectively area transverse section and=a longitudinal view ofa polarization transformer forming-part of a waveguide structure in accordance witli' the invention;

FlG. 5 illustrates therela lar-section waveguide andgacircular-section waveguide electromagnetically eoupled to one another; the elec-: tromagnetic wave pro pagating in the TE mode in the rectangular-section waveguide and in the TE mode in the circular-section waveguide;

FlG. 6 illustrates in detail a polarization transformer forming part of agwaveguide structure in accordance with'the invention;

.FlG. .7,is a =perspective view. of a-metalljelemc eluded in the polarization transformer of F lG. :FIG. 8is a partial longitudinal section through anguidc structure in accordance {with the invention;

FIGS. 9 and 10 are cross-sectional views respectively the line two 'polarization transformers Brand-4 allowing and in 'cident !linearly polarized wave 0 issuing from rectangular-section input waveguidc.-5,-t o be converted to.a circularly polarized wave 0 which is subsequently reconverted to a linearly polarizedwave 0 identical with the incidentwaveaand transmitted to a rectangular-section output waveguide 6..

These polarization transformers 3 and 4-0 tional type have been shown in a more detailed fashion in FIG.- 2. Theyrespectively comprise high-frequency waveguides 8 and 9 having a circular crossectionathese waveguides 8 and 9 being the interposed mechanical rotating joint 2 (equipped with a microwave trap 7) enaxis X,X whilc thewavegurde-S rernainsfixed. At the centers'of the waveguides -8 andi9there are re'spec;

tively arranged dielectric fins 10 and 11 serving to introduce a. differential phase shift. However, these dielectric fins l0 and 11 constitute discontinuitiesableto guides bya pair of perpendicular waveguides convey- .1; interfere with the proper operation and to-shorten the ing respective energy components polarized; -in. mutI-1 ::1

orthogonalplanes. a...

lifetime of the rotating joint. In fact, because of the difference between-the dielectric constant. of thefins and tive positions of a rectanguother polarization transformcrforming part of; awaves strueture shown in FlG. l, of a type f convenables the waveguide 9, for example, to rotate aboutthe the dielectric constant of the air generally present within the waveguide, the electric field in this waveguide can develop high values in air, while being generally of reduced level in the dielectric fins. The difference in electric-fieldstrengths gives rise to break down effects which can short-circuit the waveguide.

If, by way of example, we consider the case of a quartz fin introduced into the air-filled waveguide, then the electric-field-strength ratio in these two media is substantially 4:l, and this considerably limits the pewers which can be used in a conventional rotating-joint structure, without any risk of breakdown.

These drawbacks are eliminated in accordance with our invention by a rotating-joint waveguide including modified polarization transformers. FIGS. 3 and 4 illus- V trate a transformer of this kind which comprises a circular-section waveguide 12 provided with a series of identical metal elements 13 each of which is located in a transverse plane of waveguide 12, as FIG. 3 shows. These metal elements 13 are produced from small metal bars of rectangular section, for. example, with similar curvatures in their respective planes and with the major sides L of their cross-sectional rectangles extending in axial direction as shown in FIG. 7. The shape and position of these elements 13 in the circularsection waveguide I2, as shown in FIG. 5, are such that the curved faces of elements 13 are at right angles to the lines of force 14 of the electric field associated with the electromagnetic wave (TE mode) transmitted through the circularscction waveguide 12.

These lines of force I4 are caused by a leading component of that wave, represented by a vector E whose lagging component is represented by a vector E Elements l3 form inductive rcactances effective to advance the phase of the lagging component E so that, when the two components are in phase, there results a vector E parallel to the minor sides of an associated rectangular output waveguide 15 fed by the circular waveguide l2 as shown in FIG. II. Thus, a line s,.\' paralleling the major sides of that rectangle includes an angle of 45 with a chord a 11 subtending the acr of elements 13 and with a line perpendicular thereto bisecting that are. By the same token. a linearly polarized wave (TE,, mode) arriving over the identical input waveguide 15 in FIG. II is differentially phase-shifted to become circularly polarized. Elements 13 turn their concave sides toward the axis of waveguide 12 and have their convex sides separated from its periphery by crescent-shaped spaces; thus, their radii of curvature are larger than the radius R of the waveguide. With this curvature, as seen in FIG. 5, the elements 13 intersect substantially perpendicularly the lines of force 14 associated with field vector E The distance d separating the metal elements 13 from the center of the circular waveguide 12 (FIG. 3) is not critical but trials have led us to adopt a value of d 2R/3 for this distance.

The metal elements 13, in the embodiments of FIGS. 4 and II are disposed parallel to one another, either in an equispaced relationship or, better still, in such a fashion as to form 01 groups of n elements each.

FIG. 6 'illustrates an advantageous arrangement of L the metal elements 13 forming two groups G,, G of two elements each. This arrangement facilitates the matching of the polarization transformer. Each of these groups represents an inductive obstacle producing a phase shift of 1r/4 and these two groups are separated by 'a'distance substantially equal to Ag/4, where Ag represents the wavelength of the microwavetransmi'tted through the waveguide 12. By way of example, if the frequency of the incident wave is 3,000 MHz (corresponding to a free-space wavelength A 10 cm) and ifthe radius of curvature of the circular-section waveguide 12 is R 4 cm, the wavelength Ag in the circular waveguide 12 will be 14.70 cm and the distance betweentwo groups G and G of obstacles will be A,,/'4 3.67 cm, the distance between two consecutive elements 13 in the same group being l= 4.6cm.

The thickness e and the width L of the bars forming the metal elements 13 (FIG. 7) are chosen so that the admittance of the quadripole constituted by the polarization transformer including these elements 13 corresponds to suitable matching conditions.

Alternatively, we may provide in the circular-section waveguide "12 three groups of two elements each. In this case, the phase shift introduced by each'group of elements will be substantially equal to Ad 30.

Of course, these metal elements 13 can be also made with bars of circular section, the diameter of these bars then being determined in such a fashion as to achieve appropriate matching of the polarization transformer.

A rapid calculation shows the advantage of a polarization transformer in accordance with the invention as concerns the peak powers which can be transmitted,. lf E is the maximum electric field at the center of a circular waveguide, then the permissible peak power P is given by the relationshipz If R 4 cm and E 30 KV/cm (this value corresponding to the breakdown electric field in air), the peak power P, is 19.5 MW at a frequency of 3,000 MHz.

Assuming that the strength of the electric field between the metal elements I3 is equal to 1.5 times the strength of the electric field in a circular waveguide, the pcrmissiblepeak power P, in the polarization transformer provided with these metal elements 13 is given by:

I, 8.7 MW

The permissible peak power P, in the polarization transformer can be increased by replacing the air usually present therein by, for example, sulphur hexafluoride. v

Moreover, the metal elements 13 can readily be cooled and this makes it possible to operate the polarization transformer under very good conditions.

Also, it may be advantageous to arrange the metal elements l3 altemately at either side of a diametral plane of the circularsection waveguide 12, this plane being parallel to the delayed component E of the electric field E of the plane-polarized wave.

However, the presence of the metal elements 13 in the circular-section waveguide 12, as well as the pres- .ence of mismatching at the junctions between circular and rectangular waveguides, gives rise to reflection phenomena producing parasitic modes in the rotatingjoint structure in accordance with the invention. To

eliminate these parasitic modes, plates of resistive material (graphite, for example) can be arranged in the circular waveguide 12.

FIG. 1 1 illustrates a rotating-joint structure comprising two polarization transformers 18 and 19 separated by a rotary mechanism coupling 2. Each of these transformers l8 and 19, provided with metal elements 13, comprises a plate 20 of resistive material, respectively disposed at the input end 27 of the transformer 18 and at the output end 28 of the transformer 19, in a transverse plane, perpendicularly to the electric field E 1 of the normal TE propagation mode of the arriving mien wave 0,, the latter being linearly polarized (FIG. 12).

In some, it is desirable to be able to eliminate the phase shift introduced by the polarization transformers 18 and 12 between the incident and energent linearly polarized waves, i,e., to reverse the sense of rotation of the electric-field vector of the circularly polarized wave between waveguides 18 and 19. In this case, a phase corrector 21 (FIG. 13), constituted by a circular waveguide 22 whose axis coincides with the axis Y Y of the circular waveguide 12 and in which there are arranged metal elements 23 analogous to those of the polarization transformers l8 and 19, may be inserted between the latter, these elements 23 being arranged in relation to one another in the waveguide 22 as to produce a phase shift of 11' in the phase corrector 21. In operation, the waveguide 22 will rotate about its axis at a velocity on while the polarization transformer 19 will rotate about its axis with a velocity 2w, the transformer 18 remaining fixed.

The rotary coupling between stages 18, 21 and 19 then comprise two mechanical joints 24 and 25, each of which being provided with a microwave trap, the phase corredtor 21 being located between these two mechanical joints.

The rotating-joint waveguide structure thus obtained makes it possible to transmit large H.F. powers without producing a phase shift in the energent wave.

In FIG. 14 we have illustrated the possibility of separately injecting the components E and E of field vector E (FIG. of a circularly polarlzed wave into waveguide 12 with the aid of two conjugate rectangular waveguides a and 15b at the input end of the structure; an identical arrangement can be used, of course, for extracting these two components from such a wave guide 12 at the output end.

What we claim is:

1. In a waveguide structure for converting linearly polarized microwaves into circularly polarized microwaves and vice versa, in combination:

a first and a second waveguide of circular crosssection centered on a common axis;

joint means between said first and second waveguides enagling relative rotation thereof about said axis with uninterrupted microwave transmission there between;

input waveguide means of rectangular cross-section connected to said first waveguide for supplying same with linearly polarized microwave energy having an electric field vector in a first radial plane including said axis;

output waveguide means of rectangular cross-section connected to said second waveguide for extracting therefrom linearly polarized microwave energy having an electric field vector in a second radial plane including said axis;

first inductive phase-shifting means in said first waveguide comprising a first plurality of axially spaced elongate conductive elements of arcuate configuration connected at their ends to the inner peripheral surface of said first waveguide and spaced therefrom between said ends thereof while turning their concave sides toward said axis, a line spanning the ends of any element of said first plurality including an angle of substantially 45 with said first radial plane; and

second inductive phase-shifting means in said second waveguide comprising a second plurality of axially spaced elongate conductive elements of arcuate configuration connected at their ends to the inner peripheral surface of said second waveguide and spaced therefrom between said ends thereof while turning their concave sides toward said axis, a line spanning the ends of any element of said second plurality including an angle of substantially 45 with the said second radial plane.

2. The combination defined in claim 1 wherein the elements of each plurality are parallel'to one another.

3. The combination defined in claim 1 wherein the elements of each plurality are alternately disposed at diametrically opposite locations of the respective waveguide.

4. The combination defined in claim 1 wherein the elements of each plurality are divided into several groups spaced apart by substantially a quarter wavelength at the frequency of said microwave energy, the spacing of the elements within each group being greater than said quarter wavelength.

5. The combination defined in claim 4 wherein the relative phase shift introduced by the elements of each group between energy components polarized in said first and second planes and energy components polarized perpendicularly thereto is substantially 6. The combination defined in claim 1 wherein at least one of said waveguidemeans of rectangular crosssection comprises a pair of rectangular-section waveguides conveying respectively en ergy components polarized in mutually orthogonal planes.

7. The combination defined in claim 1 wherein said first and second waveguides are provided with respective conductor plates lying in diametrical planes perpendicular to said first and second radial planes, respeetively.

8. The combination defined in claim 1 wherein said elements are flat bars with major faces parallel to said axis.

9. The combination defined in claim 1 wherein said joint means comprises a pair of rotary joints separated by a further waveguide of circular cross-section.

10. The combination defined in claim 9 wherein said further waveguide is provided with further inductive phase-shifting means substantially identical with said first and second phase-shifting means. 

1. In a waveguide structure for converting linearly polarized microwaves into circularly polarized microwaves and vice versa, in combination: a first and a second waveguide of circular cross-section centered on a common axis; joint means between said first and second waveguides enagling relative rotation thereof about said axis with uninterrupted microwave transmission therebetween; input waveguide means of rectangular cross-section connected to said first waveguide for supplying same with linearly polarized microwave energy having an electric field vector in a first radial plane including said axis; output waveguide means of rectangular cross-section connected to said second waveguide for extracting therefrom linearly polarized microwave energy having an electric field vector in a second radial plane including said axis; first inductive phase-shifting means in said first waveguide comprising a first plurality of axially spaced elongate conductive elements of arcuate configuration connected at their ends to the inner peripheral surface of said first waveguide and spaced therefrom between said ends thereof while turning their concave sides toward said axis, a line spanning the ends of any element of said first plurality including an angle of substantially 45* with said first radial plane; and second inductive phase-shifting means in said second waveguide comprising a second plurality of axially spaced elongate conductive elements of arcuate configuration connected at their ends to the inner peripheral surface of said second waveguide and spaced therefrom between said ends thereof while turning their concave sides toward said axis, a line spanning the ends of any element of said second plurality including an angle of substantially 45* with the said second radial plane.
 2. The combination defined in claim 1 wherein the elements of each plurality are parallel to one another.
 3. The combination defined in claim 1 wherein the elements of each plurality are alternately disposed at diametrically opposite locations of the respective waveguide.
 4. The combination defined in claim 1 wherein the elements of Each plurality are divided into several groups spaced apart by substantially a quarter wavelength at the frequency of said microwave energy, the spacing of the elements within each group being greater than said quarter wavelength.
 5. The combination defined in claim 4 wherein the relative phase shift introduced by the elements of each group between energy components polarized in said first and second planes and energy components polarized perpendicularly thereto is substantially 90*.
 6. The combination defined in claim 1 wherein at least one of said waveguide means of rectangular cross-section comprises a pair of rectangular-section waveguides conveying respectively energy components polarized in mutually orthogonal planes.
 7. The combination defined in claim 1 wherein said first and second waveguides are provided with respective conductor plates lying in diametrical planes perpendicular to said first and second radial planes, respectively.
 8. The combination defined in claim 1 wherein said elements are flat bars with major faces parallel to said axis.
 9. The combination defined in claim 1 wherein said joint means comprises a pair of rotary joints separated by a further waveguide of circular cross-section.
 10. The combination defined in claim 9 wherein said further waveguide is provided with further inductive phase-shifting means substantially identical with said first and second phase-shifting means. 